The present invention relates to a tire with radial carcass reinforcement anchored on either side to at least one bead wire and having a crown reinforcement formed of at least two so-called working plies, superposed and made of wires or cables which are parallel in each ply and are crossed from one ply to the next, forming angles of at most 45xc2x0 in absolute value with the circumferential direction of the tire.
It relates more particularly to a tire of the xe2x80x9cheavy vehiclexe2x80x9d type, the ratio of the height above rim H to the maximum axial width S of which is at most 0.80, and which is intended to be fitted on a medium- or high-tonnage vehicle, such as a lorry, bus, trailer, etc.
Some current tires, called xe2x80x9chighwayxe2x80x9d tires, are intended to travel at high speed and on longer and longer journeys, owing to the improvement in the road network and the growth of the motorway network throughout the world. All the conditions under which such a tire is required to travel without doubt make it possible to increase the number of kilometers traveled, the wear of the tire being less; on the other hand, the endurance of the latter and in particular of the crown reinforcement is adversely affected.
U.S. Pat. No. 4,271,890 discloses that although the type of tire in question, with a low H/S ratio, has numerous advantages, it also has a certain number of disadvantages, such as poor adhesion of the equatorial zone of the tread or localization of high pressures in the region of the edges of the tread, which pressures are due to the shortening of the area of contact in the longitudinal direction of the tire. In order to overcome said disadvantages, the cited Patent advocates arranging between the carcass reinforcement and the radially inner working ply, in two zones distant from the equatorial plane, two limiting blocks each formed of two superposed plies of inextensible cables, crossed from one ply to the other, forming opposed angles with the circumferential direction which are at most equal in absolute value to half the smallest angle used in the working plies, and other than 0xc2x0.
The application of the solution set forth above, and the application thereof to a tire of H/S form ratio of at most 0.80, in particular at most 0.60, intended to be fitted on a heavy vehicle, does not make it possible to obtain satisfactory endurance of the crown reinforcement. The lack of endurance relates both to the fatigue resistance of the crown plies, and in particular the resistance to separation between ends of plies, and to the fatigue resistance of the cables of the portion of carcass reinforcement located beneath the crown reinforcement, the first deficiency being greatly influenced by the operating temperature at the edges of the working plies, whether traveling in a straight line or under drift.
The object of U.S. Pat. No. 5,738,740 is to improve the endurance of the crown reinforcement of a xe2x80x9cheavy vehiclexe2x80x9d tire, of a form ratio of at most 0.60, said reinforcement comprising at least two working crown plies of inextensible cables, crossed from one ply to the next, forming angles of between 10xc2x0 and 45xc2x0 with the circumferential direction, said plies having widths at least equal to 80% of the maximum axial width S0 of the carcass reinforcement. It proposes arranging, firstly between the carcass reinforcement and the working ply radially closest to the axis of rotation, an axially continuous ply formed of inextensible metal cables forming with the circumferential direction an angle of at least 60xc2x0 and the axial width of which is at least equal to the axial width of the shortest working crown ply, and, secondly between the two working crown plies, an additional ply formed of metallic elements oriented substantially parallel to the circumferential direction, the axial width of said ply being at least 0.7 S0, and the modulus of elasticity upon traction of which being at most equal to the modulus of the same name of the most extensible working ply.
Although the problems relating to the separation between working plies and the fatigue resistance of the carcass reinforcement cables would appear to be solved in one case, if the operating temperatures are greatly reduced in the other case, on the other hand prolonged travel of the tires thus constructed causes fatigue failure of the cables of the additional ply, and more particularly of the edges of said ply, whether or not the so-called triangulation ply is present.
It is always possible to change the reinforcement elements concerned, and in particular to select cables of a different construction or cables of greater tensile strength. The solution above, while admittedly simple, is still costly.
In order to overcome the above drawbacks and to improve the endurance of the crown reinforcement of the type of tire in question, U.S. Pat. No. 6,401,778 B1 has proposed a tire with radial carcass reinforcement of maximum axial width S0, comprising a crown reinforcement formed of at least two working crown plies of inextensible reinforcement elements, crossed from one ply to the other, forming angles of between 10xc2x0 and 45xc2x0 with the circumferential direction, said plies having axial widths at least equal to 80% of the width S0, and, arranged radially between said working plies, an additional ply of reinforcement elements substantially parallel to the circumferential direction, said working plies having widths greater than the width of the additional ply by at least 16% of the width S0, and, on either side of the equatorial plane and in the immediate axial extension of the additional ply, being coupled over an axial distance at least equal to 3.5% of the width S0, then being decoupled by profiled members of rubber mix at least over the remainder of the width common to said two working plies.
The choice of circumferential reinforcement elements for the additional ply of inextensible elements, such that the modulus of elasticity under traction of said additional ply is at most equal to the modulus of the same name of the most extensible working ply, and preferably low for a relative elongation of between 0% and 0.4%, and at most equal to the greatest modulus of elasticity upon traction of the most extensible working ply, for relative elongations greater than 0.4%, does not make it possible to obtain the best price/quality ratio, since the optimum fatigue strength of the circumferential elements requires a minimum density of the elements at the edges of the ply and a minimum tensile strength of said elements.
The object of the invention is to improve the endurance of the crown reinforcement of the type of tire in question, without being faced with problems of fatigue of reinforcement elements. In the context of coupling the two working plies radially on either side of the additional ply of reinforcement elements, the solution consisting in selecting said elements as being circumferential has unexpectedly not proved to be the best solution.
The tire according to the invention, with a radial carcass reinforcement of maximum axial width S0, comprising a crown reinforcement formed of at least two working crown plies of inextensible reinforcement elements, crossed from one ply to the other, forming angles of between 10xc2x0 and 45xc2x0 with the circumferential direction, said plies having axial widths at least equal to 80% of the width S0, and greater by at least 16% of the width S0 than the width of an additional ply of reinforcement elements, arranged radially between said working plies, said working plies, on either side of the equatorial plane and in the immediate axial extension of the additional ply, being coupled over an axial distance at least equal to 3.5% of the width S0, then being decoupled by profiled members of rubber mix at least over the remainder of the width common to said two working plies, is characterized in that the reinforcement elements of the additional ply are inextensible and substantially radial.
The thickness of the decoupling profiled members between working plies, measured at the ends of the least wide working ply, will be at least equal to two millimeters, and preferably greater than 2.5 mm.
xe2x80x9cCoupled pliesxe2x80x9d is to be understood to mean plies, the respective reinforcement elements of which are separated radially by at most 1.5 mm, said thickness of rubber being measured radially between the upper and lower generatrices, respectively, of said reinforcement elements. xe2x80x9cInextensible elementxe2x80x9d is to be understood to mean an element, whether cable or monofilament, which has a relative elongation of less than 0.2% when it is subjected to a tensile force equal to 10% of the breaking load. In the case of the tire in question, the inextensible reinforcement elements are preferably inextensible metal cables made of steel.
Substantially radial reinforcement elements, whether wires or cables, are elements which form angles within the range of +85xc2x0, xe2x88x9285xc2x0 around 0xc2x0 with the circumferential direction.
The working plies generally are of unequal axial widths. The radially outermost working ply may be less wide axially than the radially innermost working ply: it is then advantageous for the crown reinforcement to be finished off radially to the outside by an additional ply, referred to as a protective ply, of so-called elastic reinforcement elements, oriented relative to the circumferential direction at an angle of between 10xc2x0 and 45xc2x0 and of the same direction as the angle formed by the inextensible elements of the least wide working ply. Said protective ply may have an axial width less than the axial width of the least wide working ply, but advantageously sufficient totally to cover the zone of coupling between the two working crown plies, and even more advantageously the tread of the tire in question comprises a circumferential or quasi-circumferential groove axially arranged radially on the zone of coupling between the two working plies. Said protective ply may also have an axial width greater than the axial width of the least wide working ply, such that it covers the edges of the least wide working ply and such that it is coupled, in the axial extension of the additional ply, with the widest working crown ply over an axial distance equal to at least 2% of the width S0, then being decoupled, axially to the outside, from said widest working ply by profiled members of a thickness of at least 2 mm. The protective ply formed of elastic reinforcement elements may, in the case referred to above, be firstly possibly decoupled from the edges of said least wide working ply by profiled members of a substantially lesser thickness than the thickness of the profiled members separating the edges of the two working plies, and secondly have an axial width less than or greater than the axial width of the widest crown ply.
Whatever the solution described above, the crown reinforcement may be finished off, radially to the inside between the carcass reinforcement and the radially inner working ply closest to said carcass reinforcement, by a triangulation ply of inextensible reinforcement elements, forming with the circumferential direction an angle greater than 60xc2x0 and of the same direction as that of the angle formed by the reinforcement elements of the ply radially closest to the carcass reinforcement. Said triangulation ply may have an axial width less than said widest working ply, which, in the crown reinforcement in question, is radially closest to the carcass reinforcement. Said triangulation ply may also have an axial width greater than the width of the widest working ply, and it is then advantageous for the radially outer, so-called protective, ply coupled to the widest working ply, also to be coupled, in the immediate axial extension of the widest working ply, to said triangulation ply over an axial distance of at least 0.02 times the width S0 of the carcass reinforcement, then being decoupled, axially to the outside, from the edges of said triangulation ply by profiled members of a thickness of at least 2 mm. The protective ply formed of elastic reinforcement elements may then be decoupled from the edges of said least wide working ply, as previously, by profiled members of a substantially lesser thickness than the thickness of the profiled members separating the edges of the two working plies. Said protective ply may also be wider or less wide than the triangulation ply.
The radially outermost working ply may be axially wider than the radially innermost working ply: it is then advantageous for the crown reinforcement to be completed, radially on the inside, by a triangulation ply of inextensible reinforcement elements forming with the circumferential direction an angle greater than 60xc2x0 of the same direction of that of the angle of the reinforcement elements of the least wide ply. The so-called triangulation ply may have an axial width which is less than the axial width of the least wide working ply, that is to say, the ply radially closest to the carcass reinforcement. Preferably, said triangulation ply will have an axial width greater than the width of the least wide working ply and a width such that it is coupled, in the axial extension of the least wide working ply, to the widest working ply over an axial distance of at least 0.02 times the width S0, then being decoupled from the edges of said ply by means of profiled members of rubber mix of a thickness of at least 2 mm, and that it is the triangulation ply or the widest working ply which has the greatest width.
The crown reinforcements which have been described above, with the working ply radially farthest from the carcass reinforcement as being axially the widest, may also be completed, radially to the outside of said widest working ply, by a protective ply of elastic reinforcement elements, oriented at an angle of between 10xc2x0 and 45xc2x0 relative to the circumferential direction, of the same direction as that of the angle of the elements of the radially widest working ply. Said protective ply may have an axial width which is less than the width of the least wide working ply, and may completely cover the zone of coupling between the two working crown plies. It may also be wider than the least wide working ply, and less wide than the widest working ply, but will preferably have an axial width such that it radially covers the edges of the widest working ply, possibly being separated from said edges by profiled members of a thickness which may be less than the thickness of the profiled members separating the edges of the least wide working ply from the widest working ply, then, in the axial extension of the least wide working ply, being coupled with the radially inner triangulation ply formed of greatly inclined inextensible elements, over an axial width at least equal to 2% of the width S0, then decoupled from the edges of said triangulation ply by rubber profiled members of a thickness of at least 3 mm, whether it be the triangulation ply or the protective ply which is the widest.
The characteristics and advantages of the invention will be better understood with reference to the following description, which refers to the drawings, illustrating in non-limitative manner examples of embodiments of the invention.